Method of transmitting frames, and corresponding stations and computer program

ABSTRACT

A frame transmission method is provided for use in a multiuser MIMO system having a transmitter with a plurality of antennas and receivers that are respectively associated with users. The method includes: constructing a sounding frame in which a first portion has at least one symbol for synchronizing destination receivers and a second portion has at least as many pre-coded pilot symbols as there are destination receivers; transmitting the sounding frame in a broadcast mode for its first portion and in a directional mode for its second portion to each of the destination receivers; and constructing a respective data frame for sending to each of the destination receivers by taking account of feedback information coming from the destination receivers and, for each destination receiver, coding interference between destination receivers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Section 371 National Stage Application ofInternational Application No. PCT/FR2012/052487, filed Oct. 29, 2012,which is incorporated by reference in its entirety and published as WO2013/061009 on May 2, 2013, not in English.

FIELD OF THE INVENTION

The present invention relates to the field of telecommunications. Withinthis field, the invention relates more particularly to the field ofdigital communications by radio, also referred to as wirelesscommunications, and including WiFi networks as standardized in theInstitute of Electrical and Electronic Engineers (IEEE) standards 802.11et seq., where it is necessary to have knowledge about the channel ontransmission. Those standards guarantee interoperability betweenwireless communications devices that comply therewith.

More precisely, the invention relates to transmitting and receivingframes between WiFi devices that enable the transmitter to take accountof the propagation channel on the basis of information returned in afeedback message coming from the receiver.

The term “device” is used herein to mean an appliance forming part of abasic service set (BSS) constituted by an access point and the stationsassociated with the access point, i.e. the stations situated in thecoverage zone of the access point.

The invention is particularly applicable to multiuser multiple input andmultiple output (MU-MIMO) type transmission systems that make use ofspace division multiple access (SDMA) together with beamforming. In theMU-MIMO field, the term “user” is often used to designate the stationwith which the user is associated.

PRIOR ART

MU-MIMO transmission techniques requiring knowledge of the propagationchannel on transmission involve two stages: a feedback stage F ofrecovering information about the channel without transmitting payloaddata, and a transmission stage T of transmitting payload data, as shownin FIG. 1 for an SDMA technique in a context having three devices. Theexample shown comprises a transmission stage made up of a base element,i.e. a payload data transmission packet encapsulated in a frame followedby acknowledgments of the reception of the payload data as transmittedby the users.

The transmission stage T may include a plurality of successive baseelements so long as the channel does not vary. FIG. 2 shows atransmission stage having three base elements transmitted in succession.

During the feedback stage F, the transmitter, e.g. an access point AP,sends a packet comprising pilots, referred to as a “sounding” packet tothe stations (STA#1, STA#2). These packets enable the stations toestimate information about the state of the downlink (DL) channel.Thereafter, these stations encode the channel estimate, written CSI_DL,and transmit it to the access point AP in a channel state informationreport “CSI report” frame. This mechanism, which makes an estimate ofthe channel available on transmission, is sometimes referred to as“explicit” feedback.

The variation over time of the channel determines the duration of thetransmission stage. Thus, if the channel varies sufficiently during aperiod of F+nTe milliseconds (ms) for the SDMA technique to ceasefunctioning, e.g. because of interference between users becoming toostrong, then the duration of the transmission stage must be shorter thannTe ms. At the end of this transmission stage T, a new feedback stage Fis performed, in order to obtain an up-to-date estimate of thetransmission channel.

For example, the beamforming technique of the 802.11n standard thatserves to focus transmission power towards a particular receiver andthereby to increase the signal-to-noise ratio on reception, makes use ofa feedback stage once every 100 ms in a deployment context correspondingto an open space without the transmitters and/or receivers moving. Thisduration after which the feedback stage is performed is shortened to 25ms in the same environment when using the multiuser SDMA technique.

It is found that the longer the duration of the stage T in which payloaddata is transmitted, the greater the overall transmission rate.Nevertheless, as mentioned above, the length of the transmission stageis limited by variations in the channel over time due to movements ofthe transmitter or of the receiver or due to modifications in theenvironment.

During the transmission stage, the access point AP transmits one or morepayload data packets, written DL-MU-MIMO, to a plurality of users.Simultaneous transmission is conventionally performed by forming beamsaimed respectively at different users by means of a data precodingtechnique that makes use of the response of the propagation channel ofeach user. The best known precoding technique is the so-called “zeroforcing” (ZF) technique as set out in article [1]. ZF precoding consistsin weighting user data with respective precoding vectors determined onthe basis of the estimate of the channel for the purpose of cancelingthe interference caused by one user at the receivers of other users.

Nevertheless, performing those techniques does not always enable perfectcancellation to be achieved, in particular because of the adaptationsthat are needed during implementation. Thus, in order to reduce thesignaling overload that results from explicit feedback, sometimes onlythe best modes constitute the subject matter of the feedback. After thechannel has been estimated, the MIMO channel is decomposed into singularvalues and eigen vectors. For a 4×4 MIMO channel, there are foursingular values and four corresponding eigen vectors. Each singularvalue and its associated eigen vector represents something called a“proper mode” of the channel over which a data stream can betransmitted. The proper modes are mutually orthogonal and therefore donot interfere with one another. In the best circumstances, it is thuspossible to have four proper modes and to transmit four space-dividedstreams in parallel. However in general, certain proper modes have asingular value of zero, which means that it is not possible to transmita stream over those modes. Under such circumstances, only the strongestmodes constitute the subject matter of feedback since it is only thosemodes on which it is possible to transmit information. Under suchcircumstances, the access point obtained only partial information aboutthe channel and consequently inter-user interference cannot be canceledcompletely.

In addition, under certain conditions that are said to be “mobile”, i.e.when the devices are moving relative to one another, there may be adifference between the characteristics of the channel over which thefeedback returns and the characteristics of the channel during thefollowing transmission stage. This difference can lead to an increase ofinterference between users on reception.

Recent work developed in particular around the IEEE802.11acstandardization group, has set out to find techniques for reducing thatinterference.

Thus, with the SDMA technique, for example, an access point APpossessing four antennas can transmit simultaneously over the samefrequency band to four users, by forming respective beams pointing toeach of the users in order to eliminate interference between users, asshown in FIG. 3. These beams are calculated on the basis of the channelinformation of each user.

Under ideal situations, each user receives only the information intendedfor that user, i.e. only one spatial stream from the four spatialstreams sent by the access point.

In a first implementation, the access point sends to each user only thepilot symbols needed for estimating the channel associated with thespatial stream destined for that user. In that implementation, and withreference to FIG. 4, the access point transmits a sounding packet withomnidirectional transmission for the first fields L-STF, L-LTF, L-SIG,and VHTSIGA, and then using directional transmission for the followingfields, i.e. using precoding, in particular for the VHT-LFTi fieldscontaining the pilots. The sounding packet is encapsulated in a framethat compresses the first four fields (L-STF, L-LFT, L-SIG, VHTSIGA) andthe following four fields (VHT-STF, VHT-LTF1, VHTSIGB, VHTData) afterprecoding and then addition.

Nevertheless, in particular because of its variation over time, the realchannel departs from the ideal situation and a user can receive theinformation intended for that user together with interference frominformation intended for the other users, as shown in FIG. 5. If a userpossesses more receive antennas than there are spatial streams intendedfor that user, it is possible for the user to eliminate some or all ofthe interference by known interference canceling techniques (such as theminimum mean square error (MMSE) technique). Nevertheless, that ispossible only if the user is capable of estimating the channel for thespatial streams for the other users.

In a second implementation, the access point transmits to each user notonly the pilot symbols enabling the user to estimate the channel for thespatial stream intended for that user, but also the pilot symbolsenabling the user to estimate the channels for the spatial streams thatare for the other users, as shown in FIG. 6, thus making it possible toeliminate some or all of the interference between users.

In this implementation, the access point transmits the pilot symbolsintended for the various different users in succession. The pilotsymbols for the various users are thus positioned at successive instantswithin packets bound respectively to the various users. Either the pilotsymbols for one user are transmitted on their own, or else they aremultiplexed with the pilot symbols for the other users but in such amanner that the links are mutually orthogonal, i.e. in such a mannerthat the links between the transmitter and the various users areorthogonal. Link orthogonality is obtained by precoding the pilots withthe precoding vectors of the users.

On reception, each user estimates the channel followed by the directsignal and also the channels followed by the interference, by making useof the various pilot symbols. The estimates of the various channels arefed back to the access point by each user. Patent application WO2010/117816 A1 gives a detailed description of one construction for SDMAprecoding vectors based on feedback of the channel coefficients from thevarious stations.

That second implementation makes it possible to be more robust whenfaced with channel variations over time because it enables theinterference that is generated to be canceled. However, it requires alarger number of pilot symbols to be transmitted to each user, therebyreducing the efficiency of the physical layer of the transmissionsystem.

In any event, whatever the transmission technique, a new feedback stageneeds to be performed as soon as the interference becomes too great.

SUMMARY OF THE INVENTION

The invention proposes improving known techniques for combatinginterference by providing a frame transmission method for performing bya multiuser MIMO system comprising a transmitter with a plurality ofantennas and receivers associated with respective users.

Thus, the invention provides a frame transmission method for use in amultiuser MIMO system comprising a transmitter with a plurality ofantennas and receivers that are respectively associated with users, themethod being characterized in that it comprises:

-   -   a step of constructing a sounding frame in which a first portion        comprises at least one symbol for synchronizing destination        receivers and a second portion comprises at least as many        precoded pilot symbols as there are destination receivers;    -   a step of transmitting the sounding frame in a broadcast mode        for its first portion and in a directional mode for its second        portion to each of the destination receivers; and    -   a step of constructing a respective data frame for sending to        each of the destination receivers;

the data frame construction step taking account of feedback informationcoming from the destination receivers and, for each destinationreceiver, coding interference between destination receivers.

Thus, in the invention, the MU-MIMO transmissions are improved byenabling the transmitter to adapt its frame construction so as todiminish the impact of interference between users at the destinationreceivers.

Given its reception of sounding frames, a destination receiver usesconventional estimation techniques and the various precoded pilots toestimate its transmission channel and also the transmission channelsfollowed by the interference signals corresponding to the precodedpilots that were precoded with the vectors of the other receivers.

On the basis of its knowledge about the transmission channels, thedestination receiver calculates the power of the interference due to thesymbols for the other destination receivers. By way of example, thecalculation may be based on an average of the norm of the channelestimate over all of the subcarriers of the signals transmitted(typically using orthogonal frequency division modulation) and over allof the spatial streams.

Thus, a destination receiver that is subject to interference due toprecoded symbols for other destination receivers, interference that maybe referred to as inter-user interference or inter-receiverinterference, has means for measuring the level of the interference andfor determining which destination receivers are associated with thesymbols that are interfering with the reception of symbols intended forthe destination receiver in question.

The transmitter thus receives information from the various destinationreceivers about the inter-user interference to which each of the variousdestination receivers is subject. The transmitter can then construct thedata frame using a distribution of data for the various destinationreceivers that takes account of the inter-user interference.

In an implementation, for a given destination receiver, the feedbackinformation codes solely an identifier for that one of the interferingdestination receivers that is generating the highest level ofinterference between destination receivers from among the variousinterferences between destination receivers received by the givendestination receiver.

In this implementation, the destination receiver determines whetheranother destination receiver has a preponderant role in the interferenceto which it is subject. The destination receivers of a MU-MIMOtransmission are part of a user group. This group possesses anidentifier and each user in the group possesses a number that referencesthe channel estimation codes and the MIMO streams used (spatialstreams). During a MU-MIMO transmission, the header carries the groupidentifier information. A user can then determine from which other userthe interference comes (i.e. can determine the number of this other userin the group). It can then signal that the interference comes from theuser having the number x in the group x. The destination receiver canthus know the identifier of that other destination receiver and can codeit in the feedback information. The transmitter then has the identifierof the destination receiver that is giving rise to the most interferencein the reception of another destination receiver.

In an implementation of the invention, the data for first and seconddestination receivers is distributed in different data frames if theinterference to which one of the destination receivers is subject anddue to the other destination receiver exceeds a given threshold.

If the interference to which the destination receivers are subject thatis due to another destination receiver exceeds a given threshold and/orexceeds the interference caused by the other destination receivers, thenthe transmitter can distribute the data for the two destinationreceivers in different data frames. These frames are typicallytransmitted successively in time. Thus, the first destination receiveris no longer disturbed by the symbols for the second destinationreceiver, and the level of interference is thus greatly reduced byeliminating the major source of interference.

In an implementation of the invention, interference between destinationreceivers that exceeds a given threshold triggers a step of transmittinga sounding frame.

Alternatively, the transmitter may trigger the transmission of asounding frame. The interference levels as calculated by the destinationreceivers may take account of their knowledge of the transmissionchannels. If these transmission channels have varied since they werelast estimated, the calculated interference levels may no longer be acorrect reflection of reality. Thus, the sounding frame enables thedestination receivers to update their knowledge about the transmissionchannel.

In an implementation of the invention, the feedback information isextracted from a data acknowledgment frame coming from a datadestination receiver.

This implementation has the advantage of being compatible with numerousalready-existing WiFi devices.

In an implementation of the invention, the feedback information isextracted from a field that uses the same bits to code a modulation modeand to code a destination receiver.

This implementation has the advantage of being compatible with thelatest changes in the WiFi standards.

In an implementation of the invention, the precoded pilot symbols aredistributed in succession in the second portion of the frame.

In this implementation, a destination receiver can easily estimate thetransmission channel for the data that is intended for it, and thetransmission channels for the data that is not intended for it.

The invention also provides a WiFi device having a plurality of antennasfor a multiuser MIMO system comprising the device and receiversassociated with respective users. A WiFi device of the inventioncomprises:

-   -   means for constructing a sounding frame in which a first portion        comprises at least one symbol for synchronizing destination        receivers, and a second portion comprising at least as many        precoded pilot symbols as there are destination receivers;    -   means for constructing respective data frames for each of the        destination receivers, the data frame construction means taking        account of feedback information coming from the destination        receivers and, for each destination receiver, coding        interference between destination receivers; and    -   transceiver means for transmitting and receiving frames via a        transmission channel, these means being adapted to transmit the        sounding frame in a broadcast mode for its first portion and in        a directional mode for its second portion directed to        destination receivers.

Such a WiFi device is adapted in particular to perform theabove-described transmission method. By way of example, it may be a WiFistation or access point.

In a preferred implementation, the steps of the frame transmissionmethod of the invention are determined by program instructions in theform of one or more modules incorporated respectively in electroniccircuits such as chips which can themselves be arranged in an electronicdevice such as a WiFi access point or station. The frame transmissionmethod of the invention can equally well be performed when the program(or its modules) is/are loaded in a calculator member such as aprocessor or the equivalent, with its operation then being controlled byexecuting the program.

Consequently, the invention also applies to a computer program (or itsvarious modules), in particular a computer program on or in a datamedium and adapted to perform the invention. The program may make use ofany programming language, and it may be in the form of source code,object code, or code intermediate between source code and object code,such as in a partially compiled form or in any other desirable form forimplementing a method of the invention.

The data medium may be any entity or device capable of storing theprogram. For example, the medium may comprise storage means such as aread-only memory (ROM), e.g. a compact disk (CD) ROM, or amicroelectronic circuit ROM, or indeed the medium may comprise magneticrecording means, e.g. a hard disk, or it may comprise a universal serialbus (USB) key.

Alternatively, the data medium may be an integrated circuit in which theprogram is incorporated, the circuit being adapted to execute or to beused in the execution of the method in question.

Furthermore, the program may be converted into a transmissible form suchas an electrical or optical signal that can be conveyed via anelectrical or optical cable, by radio, or by other means. The program ofthe invention may in particular be downloaded from a network of theInternet type.

Thus, the invention also provides a computer program including programinstructions adapted to performing a frame transmission method inaccordance with the invention performed by a WiFi device for a multiuserMIMO system comprising the device and receivers associated withrespective users, when said program is loaded in and executed by theWiFi device for performing the transmission method.

The invention also provides a data medium including program instructionsadapted to performing a frame transmission method in accordance with theinvention performed by a WiFi device for a multiuser MIMO systemcomprising the device and receivers associated with respective users,when said program is loaded in and executed by the WiFi device forperforming the transmission method.

LIST OF FIGURES

Other characteristics and advantages of the invention appear moreclearly on reading the following description of particular embodiments,given merely as illustrative and non-limiting examples, and withreference to the accompanying drawings, in which:

FIG. 1, mentioned with reference to the prior art, is a diagramillustrating the feedback stage F of recovering information about thechannel without transmitting payload data, and the payload datatransmission stage T, for an SDMA transmission technique in athree-device context;

FIG. 2, mentioned with reference to the prior art, is a diagram showinga stage F of recovering information about the channel followed by atransmission stage having three base elements transmitted in succession,for an SDMA transmission technique in a three-device context;

FIG. 3, mentioned with reference to the prior art in an SDMAtransmission context, is a diagram showing an access point AP thatpossesses four antennas and that transmits simultaneously over the samefrequency band to four users by forming respective beams aimed at eachuser in order to eliminate inter-user interference;

FIG. 4, mentioned with reference to the prior art in an SDMAtransmission context, is a diagram showing a first implementation of atransmission method in which the access point transmits a sounding frameusing omnidirectional transmission for the first fields L-STF, L-LTF,L-SIG, and VHTSIGA, followed by directional transmission for thefollowing fields containing the pilots, and in particular the VHT-LTFifield;

FIG. 5, mentioned with reference to the prior art in a context identicalto that of FIG. 3, is a diagram showing the interference suffered by adestination receiver/user as a result of the information for the otherdestination receiver/users;

FIG. 6, mentioned with reference to the prior art in an SDMAtransmission context, is a diagram showing a second implementation of atransmission method in which the access point transmits a sounding frameusing omnidirectional transmission for the first fields L-STF, L-LTF,L-SIG, and VHTSIGA, and then using directional transmission for thefollowing fields containing the pilots, and in particular the VHT-LTFifield;

FIG. 7 is a flow chart showing the main steps of the transmission methodof the invention; and

FIG. 8 is a diagram of a simplified structure example for a stationsuitable for transmitting data packets in accordance with the invention.

DESCRIPTION OF AN IMPLEMENTATION OF THE INVENTION

The invention is described in the context of the IEEE 802.11ac standardas described in particular in document IEEE802.11-11/0040r0. Accordingto that document, a “MAC VHT control field” frame serves to feed backinformation, in particular for link adaptation.

B3- B6- B9- B12- B16- B0 B1 B2 B5 B8 B11 B15 B23 HT/ Solicited/ MRQ MSIMFSI/ Nsts MCS SNR VHT unsolicited GID-L

B24-B26 B27 B28 B29 B30 B31 GID-H Coding FB TX type Reserved AC RDG/moretype constraint PPDU

The field MFB enables link adaptation to be performed in MU-MIMO mode.In this mode, the station may make a proposal to the transmitter for itto adapt its transmission (data rate, . . . ) given its knowledge of thepropagation channel, the performance of its receiver, and previoustransmissions. The field Nsts defines the number of spatial streams thatthe station is requesting from the transmitter/access point. The fieldMCS defines the modulation and the efficiency of the code requested bythe station from the transmitter/access point. The field SNR defines thesignal-to-noise ratio estimated at the receiver.

This “VHT control field” frame may be included in various differenttransmissions, and in particular in acknowledgment frames.

The example described relates to a system of the kind shown in FIG. 5.The MU-MIMO system has an access point with a plurality of antennas anda plurality of receivers/users. The method 1 takes place as shown by theflow chart of FIG. 7. The method comprises:

-   -   a step 2 of constructing a sounding frame in which a first        portion comprises at least one symbol for synchronizing        destination receivers and a second portion comprises at least as        many precoded pilot symbols as there are destination receivers;    -   a step 3 of transmitting the sounding frame in a broadcast mode        for its first portion and in a directional mode for its second        portion to each of the destination receivers; and    -   a step 4 of constructing a respective data frame for sending to        each of the destination receivers.

Data frame construction step 4 takes account of feedback informationfbck coming from the destination receivers and, for each destinationreceiver, coding its interference between destination receivers.Typically, the coded interference makes it possible to identify that oneof interfering destination receivers that is generating the highestlevel of interference. The coded interference may code the interferencelevels to which the destination receiver is subject.

In a first implementation, the identifier of the interfering destinationreceiver is included in a new field “Interf” indicating that poordetection by the destination receiver is due to an interferingdestination user/receiver (Interf=1). The Interf bit is set to “0” ifreception is good or in the event of reception being poor but notbecause of interference due to another destination receiver. Another newfield “interfering user index” serves to identify the destination usercausing the interference. Given that groups of users can have a maximumof four users and that the ordering of these users in the group isknown, two bits suffice to identify a user. For example, 00: user 1, 01:user 2, . . . . The composition of the “VHT control field” frame is thenas follows:

B3- B6- B9- B12- B16- B0 B1 B2 B5 B8 B11 B15 B23 HT/ Solicited/ MRQ MSIMFSI/ Nsts MCS SNR VHT unsolicited GID-L

B24- B33- B26 B27 B28 B29 B30 B31 B32 B34 GID- Coding FB TX Reserved ACRDG/ Interf inter- H type type constraint more fering PPDU user index

In a second implementation, no field is added to the “VHT control field”frame for information feedback. The field MCS has four bits, thus makingsixteen combinations possible. Only ten combinations are identified inthe IEEE 802.11ac standard as described in document 1361r3. Fourcombinations are thus available. These four combinations enable theinterfering destination receiver to be coded in the same manner as inthe first implementation.

This second implementation does not require the “Interf” bit to be fedback since the use of any one of those four combinations implicitlycontains the information that reception by the destination receiver isstrongly disturbed by inter-user interference.

FIG. 8 is a diagram of a simplified structure example for a stationsuitable for transmitting frames in accordance with the invention.

The station is intended in particular for a multiuser MIMO system havinga transmitter with a plurality of antennas TX/RX and receiversassociated with respective users. The station acts as the transmitterand can also have the function of a WiFi access point giving access to atelecommunications network.

The station STA comprises:

-   -   means for constructing a sounding frame in which a first portion        comprises at least one symbol for synchronizing destination        receivers, and a second portion comprising at least as many        precoded pilot symbols as there are destination receivers. These        means typically comprise digital signal processor (DSP)        calculation means, e.g. a microprocessor or a DSP, that are        microprogrammed for calculating the pilot symbols precoded with        the respective precoding vectors of the destination receivers;    -   means for constructing respective data frames for each of the        destination receivers, the data frame construction means taking        account of feedback information coming from the destination        receivers and, for each destination receiver, coding        interference between destination receivers together with an        identifier of the interfering destination receiver. In the        embodiment shown, these means are the same as the means for        constructing the sounding frame, however the microprogramming of        the DSP includes modules adapted respectively for constructing        the sounding frame and for constructing the data frame; and    -   transceiver means T/R for transmitting and receiving frames via        the transmission channel. These means comprise a conventional        transceiver system adapted to transmit the sounding frame in a        broadcast mode for its first portion and in a directional mode        for its second portion directed to destination receivers. These        transceiver means T/R are connected to the sounding frame        construction means and to the data frame construction means.

Although the present disclosure has been described with reference to oneor more examples, workers skilled in the art will recognize that changesmay be made in form and detail without departing from the scope of thedisclosure and/or the appended claims.

[1] D. Gesbert, M. Kountouris, R. W. Heath, C. B. Chae, T. Sälzer,“Shifting the MIMO paradigm”, IEEE Proc. Mag., Sep. 2007.

1. A frame transmission method for use in a multiuser MIMO systemcomprising a transmitter with a plurality of antennas and receivers thatare respectively associated with users, the method comprising: a step ofconstructing a sounding frame in which a first portion comprises atleast one symbol for synchronizing destination receivers and a secondportion comprises at least as many precoded pilot symbols as there aredestination receivers; a step of transmitting the sounding frame in abroadcast mode for its first portion and in a directional mode for itssecond portion to each of the destination receivers; and a step ofconstructing a respective data frame for sending to each of thedestination receivers; the data frame construction step taking accountof feedback information coming from the destination receivers and, foreach destination receiver, coding interference between destinationreceivers.
 2. The transmission method according to claim 1, wherein, fora given destination receiver, the feedback information codes solely anidentifier for that one of the interfering destination receivers that isgenerating the highest level of interference between destinationreceivers from among the various interferences between destinationreceivers received by the given destination receiver.
 3. The frametransmission method according to claim 1, wherein the data for first andsecond destination receivers is distributed in different data frames ifthe interference to which one of the destination receivers is subjectand due to the other destination receiver exceeds a given threshold. 4.The frame transmission method according to claim 1, wherein interferencebetween destination receivers that exceeds a given threshold triggers astep of transmitting a sounding frame.
 5. The frame transmission methodaccording to claim 1, wherein the feedback information is extracted froma data acknowledgment frame coming from a data destination receiver. 6.The frame transmission method according to claim 5, wherein the feedbackinformation is extracted from a field that uses the same bits to code amodulation mode and to code a destination receiver.
 7. The frametransmission method according to claim 1, wherein the precoded pilotsymbols are distributed in succession in the second portion of theframe.
 8. A WiFi device comprising: a plurality of antennas for amultiuser MIMO system comprising the device and receivers associatedwith respective users; means for constructing a sounding frame in whicha first portion comprises at least one symbol for synchronizingdestination receivers, and a second portion comprising at least as manyprecoded pilot symbols as there are destination receivers; means forconstructing respective data frames for each of the destinationreceivers, the data frame construction means taking account of feedbackinformation coming from the destination receivers and, for eachdestination receiver, coding interference between destination receivers;and a transceiver configured for transmitting and receiving frames viathe transmission channel, the transceiver being adapted to transmit thesounding frame in a broadcast mode for its first portion and in adirectional mode for its second portion directed to destinationreceivers.
 9. (canceled)
 10. A non-transitory data medium includingprogram instructions stored thereon and adapted to perform a frametransmission method performed by a WiFi device for a multiuser MIMOsystem comprising the device and receivers associated with respectiveusers, when said program is loaded in and executed by the WiFi devicefor performing the transmission method, wherein the method comprises:the Wifi device constructing a sounding frame in which a first portioncomprises at least one symbol for synchronizing destination receiversand a second portion comprises at least as many precoded pilot symbolsas there are destination receivers; the Wifi device transmitting thesounding frame in a broadcast mode for its first portion and in adirectional mode for its second portion to each of the destinationreceivers; and the WiFi device constructing a respective data frame forsending to each of the destination receivers by taking account offeedback information coming from the destination receivers and, for eachdestination receiver, coding interference between destination receivers.